1. Field of the Invention
The present invention relates to a mass flowmeter of Coriolis type that generates a force known as Coriolis force acting in proportion to the mass flow rate of a fluid passing through a pipe by forcibly oscillating the pipe, and determining the mass flow rate by detecting a phase difference of the vibrations on the upstream side and on the downstream side of the pipe.
2. Description of the Background Art
A flow meter of Coriolis type forcibly oscillates with an oscillator a fluid pipe for passing a fluid to be measured, detects a phase difference of the vibrations generated on the upstream side and on the downstream side of a flow path in accordance with a flow rate of the mass of a fluid, and determines the mass flow rate from the phase difference.
Here, prior to the description of the problem of the present invention, a structure, a principle, and the like of a mass flowmeter of Coriolis type will be described.
In FIG. 11, for example, a substantially U-shaped fluid pipe 1 forms a flow path of a measurement fluid 100 to be measured. The measurement fluid 100 is introduced from one end of the fluid pipe 1, and passes through a bent portion and a straight-pipe portion to be ejected from the other end.
Both ends of the substantially U-shaped fluid pipe 1 are fixed to a supporting portion 101. When this is seen from a viewpoint of structural mechanics, the supporting portion 101 serving as a supporting member supports the fluid pipe 1 such that both ends of the fluid pipe 1 will be fixed ends relative to the vibration generated by the oscillation of a oscillator 2 described below, that is, it will be similar to a structure where the fluid pipe 1 is supported in a cantilever state. As a result, as shown in the model view of FIG. 13A, the fluid pipe 1 can be warped with an axis located at both ends inserted into the supporting portion 101, that is, at the fixed ends of the vibration by the oscillation or in its vicinity.
In FIG. 11, an oscillator 2 is disposed at an intermediate portion in the fluid pipe 1. The oscillator 2 is made of a permanent magnet 21 fixed to the fluid pipe 1 and an electromagnetic driving coil 22 fixed on a base 102. The permanent magnet 21 (magnetic substance) is inserted into the electromagnetic driving coil 22, and oscillates the fluid pipe 1 when an alternating current is passed through the electromagnetic driving coil 22 by an oscillation circuit 34 (FIG. 12).
On the other hand, the fluid pipe 1 is provided respectively with first and second detectors 2A, 2B. That is, the first and second detectors 2A, 2B are arranged to be spaced apart from each other on the upstream side and on the downstream side along a flow path of the fluid pipe 1. Each of the detectors 2A, 2B of FIG. 11 is made of a well-known electromagnetic pickup, and detects the velocity of the vibration which is one of the vibration states in the vertical direction of the fluid pipe 1 when a detected element 23 made of a permanent magnet (magnetic substance) performs a reciprocating movement within a coil 24.
In FIG. 12, a signal related to the velocity of the vibration that has been detected by the first detector 2A passes through one detection circuit 33 to be transmitted to a calculator 32 of a microcomputer 3 and to be transmitted to an oscillation circuit 34. The oscillation circuit 34 supplies to the electromagnetic driving coil 22 constituting the oscillator 2 an electric current that accords to the magnitude and the positive/negative direction of the signal related to the velocity of the vibration that has been detected by the first detector 2A such that positive feedback may be applied. At this time, positive feedback is applied at a specific natural frequency of the fluid pipe 1 which is a frequency at which the vibration is difficult to be damped against the oscillation to generate an oscillated state, whereby the vibration at the basic natural frequency can be maintained at a constant level. Here, by adjusting the arrangement of the first detector 2A, the positive/negative direction of the positive feedback, and the like, the vibration of the fluid pipe 1 can be maintained at a specific frequency of higher order selectively from a plurality of the natural frequencies of the fluid pipe 1.
When a setting is made to oscillate at the basic natural frequency, the fluid pipe 1 vibrates while warping up and down in the order of the one-dot chain line L1, the solid line LO, and the two-dot chain line L2 of FIG. 13A by the oscillation.
On the other hand, by the oscillation and the flow of the measurement fluid 100 (FIG. 11), a force known as Coriolis force acts on the fluid pipe 1, whereby the fluid pipe 1 warps and vibrates up and down while being twisted as shown in FIG. 13B.
The magnitude of the Coriolis force is proportional to the mass of the fluid that flows through the fluid pipe 1, the velocity thereof, and the angular velocity of the oscillation, and the direction of the force coincides with the direction of the vector product of the movement direction (velocity vector) of the fluid and the angular velocity at which the fluid pipe 1 is oscillated. The flow direction of the fluid will be opposite on the entrance side and on the exit side of the fluid in the fluid pipe 1. For this reason, a torque of twist is generated in the fluid pipe 1 by the Coriolis force. This torque changes with the same frequency as the oscillation frequency, and the amplitude value thereof will have a predetermined relationship with the mass flow rate of the fluid.
The warpage of the fluid pipe 1 of FIG. 13A by the oscillation and the twist of the fluid pipe 1 of FIG. 13B by the Coriolis force are superposed on each other. The calculator 32 of the microcomputer 3 of FIG. 12 calculates the mass of the measurement fluid 100 that passes through the flow path of the fluid pipe 1 based on the phase of the amplitude of the twist, that is, the phase difference of the velocity signals of vibration at respective positions constituting information of each vibration detected by the detectors 2A, 2B and the detection circuits 33, 33.
Incidentally, a flowmeter is conventionally known in which a pair of fluid pipes 1 is provided, and the pair of fluid pipes is arranged in mirror symmetry so as to face each other (See U.S. Pat. No. 4,756,198, front page, Japanese Patent Application Laid-open No. 11-337383, FIG. 7, and Japanese Patent Application Laid-open No. 2003-207380, FIG. 1).
The flowmeter such as disclosed in U.S. Pat. No. 4,756,198, in which a coil is provided between a pair of fluid pipes and a magnetic substance core is provided in each of the fluid pipes and arranged in ideal mirror symmetry, can be expected that the vibration of the pair of fluid pipes will be an ideal sound or vibration by oscillation to achieve mirror symmetry, to a greater extent than a flowmeter in which a coil is provided in one of the fluid pipes and a magnetic substance core is provided in the other.
However, since the coil is disposed at a position at which the pair of fluid pipes can easily vibrate, the coil is disposed at a position that is spaced apart to some extent from a base to which the pair of fluid pipes is fixed. As a result, the coil will be supported in a cantilever state. The coil supported in a cantilever state will be not only likely to be vibrated by external turbulence vibration, but also receive a counter force of the fluid pipe due to the production errors and the like, whereby the vibration generated in the coil will be a cause of decrease in the precision. In order to alleviate the vibration generated in the coil, it is considered that the scale for achieving higher rigidity of the cantilever supporting portion is increased; however, increase in the scale and increase in the mass of the flowmeter are inevitable, thereby deteriorating the advantages of providing a pair of fluid pipes.
Therefore, a main object of the present invention is to prevent decrease in the measurement precision and to achieve scale reduction of a flow meter in the flowmeter of Coriolis type having a constitution of ideal mirror symmetry.
Incidentally, when a pair of fluid pipes is brought close to each other, the two fluid pipes undergo resonance, and the vibration states thereof will be likely to be equal to each other, so that the measurement precision will be improved. However, when the pair of fluid pipes is brought close to each other, the assembly work of fixing the magnetic substance to the fluid pipes will be difficult.
Therefore, another object of the present invention is to facilitate or enable the assembly work of a flowmeter having a reduced scale.